Finder 77.21.9.024.8250 – 77 Series Modular solid state relays

Why Choose the Finder 77.21.9.024.8250 Solid State Relay?

The Finder 77.21.9.024.8250 is a 25 A modular solid state relay (SSR) designed for contactless switching of single-phase AC loads in industrial automation, electrical heating and machinery control applications. Part of the Finder Series 77, this relay combines a low-voltage DC control input with semiconductor switching technology and a compact DIN rail mounting arrangement.

Featuring a 4–32 V DC control input and a 19–305 V AC load switching range, the Finder 77.21.9.024.8250 provides an interface between compatible DC control equipment and AC electrical loads. Its single normally open (1 NO / SPST-NO) output supports switching currents up to 25 A, subject to the manufacturer's thermal derating and installation requirements.

The relay uses zero-cross switching, which initiates conduction near the AC voltage zero crossing to help reduce switching disturbances with suitable loads. Its integrated heatsink, 22.5 mm modular housing and IP20 protection rating make it suitable for installation in professionally designed industrial electrical control panels.

Key Advantages

• 25 A maximum load current for appropriately rated AC electrical loads.
• 4–32 V DC control input for compatible PLC outputs, controllers and DC control circuits.
• 19–305 V AC switching range for single-phase AC load control.
• Zero-cross switching helps reduce electrical switching disturbances with suitable loads.
• Contactless semiconductor operation eliminates mechanical contact bounce.
• Silent switching without mechanical relay contact noise.
• High switching endurance without mechanical contact wear.
• Integrated heatsink supports thermal management during operation.
• 22.5 mm housing width for space-efficient electrical panel layouts.
• 35 mm DIN rail mounting for convenient integration into control cabinets.
• 6 kV input-to-output impulse insulation for electrical separation between control and load circuits.
• IP20 protection rating for installation within suitable electrical enclosures.

Key Features

• Finder Series 77 modular solid state relay.
• Type 77.21 single-phase switching device.
• Maximum load current: 25 A.
• Control input voltage: 4–32 V DC.
• Switched load voltage: 19–305 V AC.
• Output configuration: 1 NO (SPST-NO).
• Switching method: Zero-cross switching.
• Output technology: TRIAC semiconductor switching.
• Integrated heatsink and protective plastic housing.
• Input-to-output impulse insulation: 6 kV (1.2/50 µs).
• Silent, spark-free and bounce-free switching.
• Control and load terminals positioned on opposite sides.
• 22.5 mm housing width.
• 35 mm DIN rail mounting in accordance with EN 60715.
• Box-clamp screw terminal connections.
• IP20 degree of protection.
• Operating ambient temperature: −20 °C to +80 °C.

Reliable 24 V DC-Controlled Switching for Industrial Automation

The Finder 77.21.9.024.8250 provides electronic switching of AC loads using a semiconductor output rather than mechanically operated contacts. A compatible DC control signal within the 4–32 V DC input range activates the relay, allowing appropriately rated AC loads to be controlled by PLC outputs, electronic controllers and industrial automation equipment.

Its zero-cross switching initiates output conduction close to the AC voltage zero crossing. This is particularly useful for suitable resistive heating loads and other applications where reducing switching transients is desirable. The contactless design also eliminates mechanical contact bounce and switching noise, making it useful in applications involving repeated switching operations.

The integrated heatsink assists with heat dissipation, but the 25 A maximum load current must be assessed against ambient temperature, installation conditions and load characteristics. Suitable upstream protection, conductor sizing, ventilation and manufacturer-specified derating must be considered. As with other TRIAC-based solid state relays, a small leakage current can remain when the output is switched off, so the device must not be used as a substitute for a safety isolator.

Typical Applications

• Industrial electric heating control systems.
• Temperature-controlled resistance heating equipment.
• Industrial ovens and process heating installations.
• PLC-controlled AC load switching.
• Industrial automation and machinery control panels.
• Solenoid and contactor coil driving circuits.
• Packaging and manufacturing machinery.
• Production line electrical switching systems.
• Compatible lighting and lamp switching applications.
• Repeated switching of appropriately rated AC loads.
• OEM machinery and electrical panel manufacturing.
• Applications requiring silent, contactless switching from a DC control signal.